As is known in the art, one type of semiconductor device is a photodiode as shown in FIGS. 1A and 1B. As is also known in the art, such devices are typically used for converting amplitude modulated light signals in fiber optics to electrical currents which represent digital or analog information. For example, data might be modulated on a light source at a rate of 40 billion binary digits/sec (40 Gb) with a “1” being represented by the light being on and a “0” represented by the light being off. Since the photodiode's current output is proportional to the intensity of illumination on the photodiode, ideally, the current waveform will be a representation of the digital data carried by the light.
These photodiodes, such as photodiode 1 in FIGS. 1A and 1B convert light, having a wavelength near 1.55 μm, to hole-electron pairs (carriers) which result in an electrical current, Id flow between the anode pad 2 and the cathode pad 3. Under illumination, hole-electron generation occurs mostly in the absorption layer 18. In order to efficiently convert 1.55 μm wavelength photons to carriers, the absorption layer 18 must have a bandgap which is less than that of the photon energy. During operation, the photodiode's anode 2 is biased at a negative voltage with respect to the cathode 3. This bias voltage, Vd, results in an electrical field which separates the holes and electrons. Up to a point, the higher the Vd and the electrical field, the faster the electrons and holes (carriers) move. High carrier velocity is needed for high switching speed implying the need for a higher Vd. However, if Vd is too high, the diode can break down resulting in burnout and/or a large component of Id that is unrelated to illumination, i.e. dark current, Idark. In any event, Idark normally increases with Vd. Photodiodes, used here for 1.55 μm wavelength detection, use an In.53Ga.47As absorption layer to provide a good compromise between absorption (i.e., relatively high In content) and good breakdown and low dark current characteristics (i.e., relatively low In content).
Photodiodes are rated according to several figures of merit. These include responsivity, R; dark current, Idark; and breakdown voltage, Vbr. In addition, when illumination levels are changing rapidly e.g. the light source is switching on and off at a very high rate, say in a 40 Gb optical data link; the time required for the photodiode current to change with the illumination might be a significant portion of the optical pulsewidth, Tw. The ability for the photodiode's current to keep up with rapidly changing illumination levels is related to the 3 dB bandwidth of the photodiode, fc. When the illuminating light source power amplitude is varied sinusoidally with time at a frequency, fop, the diode current (Id) also varies sinusoidally in accordance with the illumination. Let Pac be the variation of the optical power level where Pac=Pmax−Pmin where the optical power varies between a high of Pmax to a low of Pmin. Also let Idac=Idmax−Idmin where Idac is the difference in the high (Idmax) and low (Idmin) values of Id. Also define the responsivity, Rac, as Idac/Pac. If Pac is held constant, Idac will remain nearly constant for low values of fop. However Idac and Rac fall as fop approaches fc. At fop=fc, Rac is 0.707 times its value at low fop, i.e. Rac is 3 dB down from its low-frequency value. A high fc is desirable because this implies that the photodiode current is able to follow the rapid changes in illumination level required for high-speed data and/or analog optical links. High values of Rac are desirable because this reduces the optical power, Pac needed to produce diode current, Idac.
Dark current (Idark) is the photodiode current measured with no illumination. It is desirable to minimize Idark because it contributes an error signal and noise to the Id when the photodiode is used to detect light. A major component of dark current arises when generated holes and electrons are separated by the electric field due to the anode bias and appear at the photodiode terminals as current. Idark sources can also arise from conductive leakage paths on the diode's surface.
Idark has two physically contributing effects namely surface current, Isdark and bulk current, Ibdark, such that Idark=Ibdark+Isdark. Ibdark is the dark current that originates from within the absorption layer 18 and Isdark is that which originates from the photodiode's sidewall surface 6. While a small amount of Ibdark is also generated from the natural hole-electron pair production processes in those portions of the absorption layer that are flawless, crystal defects in the absorption layer 18 and surface 6 act as generation centers which are prolific sources of hole-electron pairs. Isdak can also arise from surface 6 contamination and leakage. FIGS. 1A and 1B show respectively the cross-sectional and top views of a metamorphic photodiode 1 on a GaAs substrate 12. However, these photodiodes could also be grown on InP substrates. The current state of semiconductor technology allows the growth of high-quality photodiode material using both lattice-matched and metamorphic substrates. This means that the majority of dark current, especially in small-diameter photodiodes, is due not to bulk effects, but rather the sidewall surface 6 of the photodiode 1. The sidewall surface 6 contributes most of Idark, especially in small devices and is highly dependent on the fabrication technology used to make the photodiode 1. For instance, if the photodiodes' anode mesas 4 are formed using dry etching, the sidewall surface 6 often comes out with large numbers of crystal defects (damage) due to its exposure to the energetic gas ions inherent in dry etching processes. This damage results in high Isdark and Idark. Not only are the high values of dark current undesirable but contaminated and/or damaged sidewall surface 6 can also result in sudden failures due to anode to cathode short circuits which appear under anode bias.
Alternatively, the photodiodes' anode mesas 4 can also be formed using water-based etchants, i.e. wet etching processes. Wet etching results in an insignificant level of damage to the sidewall surface 6 and might help to reduce Isdark as a result. Unfortunately, oxidation and contamination of the photodiode surface 6 can also contribute to Isdark. Also, wet etching is generally isotropic, meaning that the etch rate is the same in all directions. The resulting undercut during etching makes it difficult to fabricate photodiodes which have a small L1 (FIG. 1A) relative to the anode mesa 4 thickness, Lt. It is desirable to minimize L1/Lt, especially for small photodiode diameters (Ld) because the photodiode area represented by L1 does not contribute to photo-generated current, Idac, but does increase the photodiode's capacitance and hence reduces the photodiode's fc and its ability to follow rapid changes in illumination.
Therefore, dry etching is often used because it etches vertically with very little undercutting and sideways etching, allowing minimization of L1. Therefore, some kind of sidewall surface 6 treatment must be incorporated in the diode process to remove the damaged material, oxidation, and/or surface contamination, so as to obtain acceptable values of dark current, Isdark.